What is tomato blossom end rot?
Blossom end rot is a dark, flat patch on the tip of the tomato, where the flower was, and it shows mostly on the first fruits when soil moisture swings. Calcium reaches the fruit with the water: if watering is uneven it does not get there, and eggshells in the soil do not change that.
By region
| North | Centre | South | |
|---|---|---|---|
| When it shows | On the first fruits, in the first hot spell, if spring watering was uneven. | As in the north, in the dry spells of summer. | More often: the heat lasts longer, and a pot can dry out in a day. |
| What to do | Keep the soil evenly moist, mulch once the ground is already warm, water the soil. | As in the north. | Water the soil more often, mulch, and don't leave a pot dry for long. |
The disorder follows the water, not the calendar. It shows sooner in a pot than in the ground.
University of Minnesota Extension describes it as a fruit disorder, common on the first tomatoes and more on large fruit than on cherries. Later fruit on the same plant can be sound if the water turns steady. Sowing and planting months are in when to plant tomatoes.
How to recognise it
The patch starts at the tip, opposite the stalk: leathery at first, then black and sunken. It often shows when the fruit is between a third and half grown. Fungi and bacteria can move in afterwards, and the fruit rots past the patch. Brown blotches that spread over leaves, stems and fruit in cool, wet weather are late blight: a different problem, under pests and diseases.
Water and calcium
University of Minnesota Extension writes that the fruit can be short of calcium even when the soil has plenty. It enters through the root tips and travels with the water. Moisture swings, too much nitrogen, damaged roots and heat that pushes the plant to grow too fast all interrupt that. A lot of potassium, magnesium or ammonium in the soil uses the same route as calcium.
Eggshells do not fix it: the same extension service explains that they break down too slowly, and that most soils already have enough calcium. Lime and gypsum are calcium sources, but only after a soil test. Water the soil in the morning, steadily: how much is in watering, and the hardware is in drip irrigation.
What to do with affected fruit
A fruit with the patch does not recover. Pick it off, so the plant puts its effort into sound fruit.
For fruit still to come: keep the soil evenly moist, mulch once the ground is warm, and don't hoe close to the stem. University of Minnesota Extension says not to cultivate within about 30 cm (1 ft) of the plant. Depth and materials are in mulching. The RHS, on tomatoes, leaves a gap around the stem, because damp mulch against the neck rots it. In a pot the compost dries out fast: see the container garden. If one variety does this every year, choose another next year.
In the app
The photo check lets you snap a plant and get an indication of its health. It is only an indication, not a firm diagnosis: for serious cases, ask a professional.
Frequently asked questions
Does blossom end rot spread to other plants?
No. It is a disorder of that fruit, tied to water and to calcium not reaching the tip. It does not spread the way late blight does.
Do eggshells fix it?
No. They break down too slowly, and the soil usually has enough calcium already. Test the soil before you add lime or gypsum.
Can the fruit recover?
No. The patch stays. Pick the fruit off. Later fruit can be sound if you keep the soil evenly moist.
Does it only hit tomatoes?
No. University of Minnesota Extension also reports it on peppers and courgettes, with the same cause: uneven water, and calcium not reaching the fruit.
What do I do tomorrow?
Water the soil, and don't let it swing from sodden to dry. Mulch once the ground is warm, clear of the stem. Don't hoe close to the roots.